Spray forming is a near-net-shape casting technology based on the atomization of a liquid stream and subsequent deposition on a substrate. Rapid solidification occurs during the spray forming process, resulting in the beneficial effects of a refined microstructure and compositional homogeneity. The process is suitable for a wide range of metallic and non-metallic materials, including low-carbon steel.
One type of spray forming technique employs gas impingement as an atomization-inducing force. Another type of spray forming avoids gas impingement by relying on the creation of a Lorentz force for atomization. The Lorentz force is created by simultaneous passing the magnetic field and an electric current through a fluid in order to create a magnetohydrodynamic (MHD) force. An apparatus and MHD spraying process is described in U.S. Pat. No. 4,919,335 to Hobson et al, which is incorporated herein by reference. The disclosure of the aforesaid patent describes a process in which an electric current is applied through molten metal while, simultaneously, a magnetic field is applied to the molten metal in a plane perpendicular to the electric current. The molten metal forms into droplets which flow in a direction perpendicular to both the electric current and the magnetic field. A nozzle described in U.S. Pat. No. 4,919,335 includes two hollow tubes which converge at a gap. Electrodes of a D.C. power source are coupled respectively to the two tubes so that a D.C. current flows through the gap when molten metal flows from the two tubes into the gap. Magnetic poles are placed in front of and behind the gap to create a magnetic field perpendicular to the direction of the electric current.
While the aforementioned nozzle is capable of effective operation in a MHD atomization spray forming operation, the electrodes are open so that metering of the molten material is difficult. If the flow of molten material poured between the two electrodes is excessive, compared to the flow capacity of the atomization electrodes, overflow of the molten metal occurs, thus seriously damaging the surrounding ancillary components. On the other hand, if the tundish flow is insufficient, intermittent and unsteady atomization will take place.
Another draw back to the aforementioned nozzle is that it has no preheating mechanism. Operation with high melting point metals, i.e., steel, requires careful attention to preheating of the spray electrodes above the melting point of the material to be atomized without overheating of close-proximity component such as the magnet and induction coils. Thus, a need exists for an improved nozzle for use in a MHD spraying process.